Therm-a-Rest Boost 650 -6C review: A luxurious transformer of a sleeping bag

The Therm-a-Rest Boost 650 -6C down sleeping bag goes from campsite dream to snug-fitting backpacking sidekick by simply zipping up the extended centre panel. It’s an innovative and versatile three-season companion.

Though still best known for its sleeping mats, Therm-a-Rest also makes some of the best sleeping bags in the business. Various models in the brand’s Hyperion, Parsec and Questar series have all performed well in previous Trail magazine and LFTO group tests over the years, even scooping a couple of coveted ‘Best in Test’ awards.

Of the three, the Hyperion range is the premium product, using 900 fill power down for an incredible warmth-to-weight ratio. Unsurprisingly, they’re also rather pricey, employing a highly tapered fit for maximum thermal efficiency.

Next are the slightly lower spec 800FP Parsec bags, and then the more affordable Questar range (using 650FP down, with a roomier cut).

Now, there’s a fourth family to welcome into the Therm-a-Rest home: the new Boost series. Like the Questar, these bags also use 650 fill power down but are designed to be more comfortable and more versatile than any other down bag that Therm-a-Rest makes.

The secret sauce is an unusual, zippered centre baffle that changes the overall profile of the bag. Unzip it and you get a roomy, semi-rectangular bag for maximum sleeping comfort, perfect for relaxed campsite use.

But on chillier nights, you can zip it up to create a slimmer, mummy-style silhouette. This reduces the ‘dead space’ inside the bag for improved thermal efficiency. According to the brand, this ‘WarmZip’ feature can boost the bag’s warmth by up to 6°C.

When it comes to performance, this gives it an EN/ISO certified temperature rating of 0°C (comfort) and -6°C (lower limit). It’s a clever design in terms of both comfort and versatility.

On the scales, the regular size weighs in at 1.21kg and packs down to dimensions of 32cm x 19cm in its compression stuff sack. There are also short and long lengths, as well as multiple extra wide versions available.

It’s one of the most inclusive size ranges we’ve seen in any sleeping bag and means there ought to be a Boost to suit you regardless of your build or body shape. It’s not the lightest or most compact option available, but it remains backpacking-friendly, especially for those who value comfort over ultralight specs.

It has enviable eco credentials, too, using 100% recycled nylon and RDS-certified insulation. Practical added extras like a snug hood, armholes, internal pocket, and zipper baffles enhance usability, though the absence of Therm-a-Rest’s Synergylink mat straps and Toe-asis foot pocket (two things we loved about the Parsec and Questar bags) are small drawbacks.

Still, this bag is ideal for relaxed adventures including car camping, travel, trekking trips and less hardcore backpacking, it delivers excellent adaptability and cosy sleep – especially for those who dislike restrictive bags – while still packing down reasonably well considering its warmth and comfort. Just don’t expect ultralight performance from this luxurious transformer of a bag.

Temperature ratings

Therm-a-Rest is originally a US brand but has had a strong UK and European market presence – with a long-established factory in Midleton, Ireland – for decades now. So, helpfully it clearly gives temperature ratings of its sleeping bags and quilts in both fahrenheit and celsius (centigrade), which is very handy for British buyers.

As its name suggests, the Boost 650 20F/-6C has an EN/ISO certified lower limit of 20°F or -6°C (defined as “the temperature at which a standard man can sleep for eight hours in a curled-up position without waking and feeling cold”).

The bag’s comfort rating is 32°F or 0°C (defined as “the temperature at which a standard woman can expect to sleep comfortably in a relaxed position”).

This means that the Boost 650 -6 is a three-season sleeping bag, suitable for use from spring through to autumn, and unless you’re a particularly cold sleeper, you’ll likely be warm enough even if overnight temperatures dip below freezing.

Having said that, the Boost is not quite as warm as two other bags we included in our 2025 group test, the premium Mountain Equipment Xenith III or the more comparable Nemo Disco 15.

On the other hand, if this 20F/-6C version of the Boost 650 is overkill for your needs, there’s also a lighter and more packable 32F/0C version (comfort 5°C, lower limit 0°C) that works better for two-season use, if you don’t expect temperatures to drop below freezing.

Insulation type, fill power and fill weight

The bag is filled with 650g of 650 fill power duck down (regular size), certified by the Responsible Down Standard. The down is also treated with a hydrophobic finish, a PFC-free water-repellent coating that means it absorbs 90% less water and dries three times faster than untreated down.

This should help to guard against general dampness – though as with all down gear, it’s good practice to try and keep your down dry, as a soaking wet down bag makes for a cold and sleepless night. Still, it’s a useful additional measure of weather resistance that means you won’t be totally screwed if your sleeping bag gets a bit wet.

As with most Therm-a-Rest down bags, the insulation is also zoned. This means that more fill is positioned in critical areas, such as the torso, to improve overall warmth.

The bag uses lateral box-wall baffles on both the topside and underside, which is a tried and tested construction method albeit not quite as sophisticated as the angled or offset baffles used by some other manufacturers.

Of course, 650 fill power down doesn’t loft as well as higher fill power down, giving the Boost a slightly lower warmth-to-weight ratio than some 700FP+ bags. But there’s plenty of fill here to ensure ample warmth in conditions down to freezing point or just below, depending on how cold you sleep.

Pack size and weight

The regular size bag weighs in at 1.21kg (2lb 11oz), with packed dimensions of 32x19cm (12.5x7.5in) when stuffed into its compression bag. So, overall weight and packed size is competitive, being very similar to 650FP rivals like the Nemo Disco 15, though not quite as light or packable as the Mammut Comfort Down -5.

Still, the Boost is by no means too big to be a viable choice for backpacking or wild camping. Sure, it’s not ultralight, but it’s still far more portable and packable than a synthetic alternative.

Materials and sustainability

The shell and lining fabrics of this bag are both made from 20-denier nylon ripstop finished with a PFC-free durable water repellent (DWR) treatment, which does a decent job of keeping general dampness from seeping into the bag’s down fill.

These fabrics are also 100% recycled as certified by the GRS (global recycling standard). They’re soft and comfortable too, with a pleasant next-to-skin feel. All in all, the bag scores highly for its environmental credentials, using ethically sourced duck down fill and 100% recycled fabric for both the shell and lining.

Size and shape

Here’s where the Boost 650 really shines. It’s an extremely comfortable and versatile bag when it comes to overall size and shape. This is partly down to its adaptable design, combined with the fact that it comes in no less than six different sizes.

You can get a short version suitable for users up to 168cm tall (5ft 6in), a regular size for those up to 183cm (6ft) and a long length for users up to 198cm (6ft 6in). In addition, there are wide versions of all three lengths.

In all cases, the cut is an even more generous evolution of Therm-a-Rest's ‘W.A.R.M. fit’ profile (‘With Additional Room for Multiple positions’), as used in its Questar series of sleeping bags. That bag was a relaxed mummy shape, intended to accommodate various natural sleep positions while maintaining thermal efficiency.

The Boost goes a step further. If you unzip the centre zipper, the silhouette isn’t really a mummy at all – it’s a semi-rectangular cut that gives you loads of room to starfish.

By way of example, in this configuration, the regular size has a shoulder girth of 163cm (64in), a hip girth of 152cm (60in) and a foot box girth of 155cm (61in). But the wide version of the bag adds another 30cm (12in) to the shoulder girth and 36cm (14in) to the hip girth. That gives people with broader hips or shoulders a viable technical sleeping bag for camping and backpacking.

If you want more details, the dimensions are all clearly stated in the ‘tech specs’ tab of the Boost 650 product listing on Therm-a-Rest’s website, which makes it easy to check which size will best fit you.

This kind of accurate size guide is something that inclusive outdoor organisations like Every Body Outdoors have been campaigning for, so it’s great to see Therm-a-Rest listening to consumers.

With the centre WarmZip fully zipped up, however, the Boost takes on a more conventional mummy shape. But even in ‘mummy mode’ we didn’t find it too narrow or restrictive.

It also means the centre baffle behind the centre zip hangs down inside the bag, which further reduces dead space between the legs for added thermal efficiency (as well as feeling nice and cosy).

Features

The stand-out feature is that central WarmZip, which is genuinely innovative and well designed. It’s not quite unique – Snugpak’s Softie Elite series has a similar side expander panel to give you more room and thermal adaptability.

However, those are synthetic bags, so the Boost is the first time we’ve encountered the concept in a down bag, and we have to say it works very well.

The bag has plenty of other features too. Up top, there’s a snug-fitting hood with a drawcord closure and a collar press-stud, plus a padded neck baffle for improved thermal efficiency.

The two-way side zipper extends right down to the footbox and has a smooth-running, anti-snag slider to stop it from getting stuck on the thin fabrics. It’s backed with a full-length insulated baffle, reducing cold spots.

Inside the bag is a small pocket, ideal for stashing a phone to keep it warm and prevent battery drain. You also get two zippered arm holes, which can be opened and closed from inside or outside the bag.

This ensures it’s easy to operate the central WarmZip, or stick your arms out if you wanted to read a book, play cards or sip from your camping mug without leaving the warmth of your sleeping bag. Obviously, they can also be used for ventilation on milder nights, so they’re a versatile feature.

On the other hand, the Boost lacks a couple of features that we’ve come to associate with Therm-a-Rest bags, which is a shame. A notable omission is the Synergylink connectors, which are elastic straps that connect the bag to a sleeping mat, preventing you from rolling off in the night.

Nor do you get the cosy ‘Toe-asis’ foot pocket that we loved on the Questar and Parsec bags. This is an integrated footwarmer inside the bag that eliminates some of the dead space at the bottom of the bag to keep your feet warmer. It works really well – ideal for those with poor circulation – and we missed not having it.

Verdict

The Therm-a-Rest Boost 650 -6C is comfort-focused down bag with more versatility than most thanks to its ability to toggle between roomy rectangular and more efficient mummy modes. Add in the practical features and you’ve got a great bag for car camping, travelling, trekking and easier backpacking or wild camping adventures.

If minimal weight and a tiny packed size are your priorities though, check out our favourite lightweight sleeping bags. Or if it's a little too warm for your needs, you might be better off with a summer sleeping bag.

About the author

Matt Jones is a freelance journalist based in the heart of Snowdonia National Park, he’s a vastly experienced gear tester and self-confessed outdoor kit geek.

Matt’s been one of our main gear testers for the last couple of years and is the first person we call with any complicated kit queries that need in-depth and forensic analysis.